1. Field of the Invention
The present invention relates to an optical communication device and methods of using this device. In particular, the present invention relates to an optical switching apparatus suitable for switching and outputting optical signals received from a plurality of optical transmission lines to other optical transmission lines, and methods for using this apparatus.
2. Prior Art of the Invention
To handle the sudden increase in data traffic through the Internet, etc. and the quickly growing demands for multimedia communication of images, sound and data, much progress has been made to increase the speed and the capacity of the transmission lines and telecommunication network nodes. To achieve a higher transmission speed, optical communication devices and optical fiber transmission lines are generally used to transmit signals between telecommunication network nodes.
In recent years, to handle the ever increasing speed of communication networks and to improve the capacity of communication devices, these communication networks and devices use optical switching apparatuses such as optical cross-connects (hereafter, referred to as OXC) and optical add-drop multiplexing apparatuses (hereafter, referred to as OADM), which implement switching processes such as switching of transmission lines and switching of circuits without converting optical signals to electric signals before processing the signals as in the conventional communication devices.
The OXC or OADM typically includes optical switches as its main components. At present, since a single stage high-capacity optical switch is not commercially available, a high-capacity optical switch is usually implemented through a multi-stage combination of the commercially available low-capacity optical switches such as 2×2 or 8×8 switches. The optical signal power loss and differential loss among the channels of a commercial low-capacity optical switch might reach from several dB to more than ten dB. These losses between the channels might be even larger for a high-capacity switch including a multi-stage combination of the commercially available low-capacity optical switches. Typically, an optical communication system includes optical transmitters and optical receivers before and after optical switches. Since these optical transmitters and receivers have limited optical transmission output powers, sensitivities and dynamic ranges, compensation is generally required for the optical switch loss and differential loss between the channels.
Several methods have been proposed to solve this problem. In “A Frequency Multiplexed Routing and Selecting Hybrid Switch,” Denshi Joho Tsushin Gakkai [Electronic Information and Communication Association]/Tsushin Society Taikai [Communication Society Conference (1999)]/B-12-17 (Reference A), a method is disclosed to compensate for the losses by placing optical amplifiers in the middle and/or at the output of the multi-stage optical switches. In “Power Control in ADM Node Using High-speed Compact-size Optical Spectrum Monitor,” Denshi Joho Tsushin Gakkai [Electronic Information and Communication Association]/Tsushin Society Taikai [Communication Society Conference (1997)]/B-10-101 (Reference B), it is disclosed that a wavelength-division-multiplexed (WDM) optical signal is first wavelength-demultiplexed by an OADM into an optical signal with multiple wavelengths, and that after controlling the optical amplitude for each of the wavelengths using variable optical attenuators, the signals are again wavelength-division-multiplexed. In this method, the amplitude for each wavelength is controlled based on the results of multiplex signal spectrum monitors after wavelength-division-multiplexing.
Kokai Patent Journal No. HEI 11 [1999]-32010 (Reference C) to the inventor of the present application discloses an OXC containing several optical switches and a few optical amplifiers between the optical switches, wherein the optical signal amplitude is controlled using a configuration wherein the amplification of optical signals is adjusted with the optical amplifiers, which is in turn controlled by the amplitude of the output optical signals.
At present, a high-capacity optical switch is usually realized by combining commercially available low-capacity optical switches in multi-stages. Therefore, it is necessary to appropriately calibrate and install an optical transmission line from the output port of an optical switch at one stage to the input port of another optical switch at the next stage. Thus, maintenance is often required for those transmission lines between the stages, and the optical transmission is interrupted during the maintenance. Further, the interruption may also occur when the high-capacity optical switch is under the normal operation.
A high-capacity switching apparatus, in which optical amplifiers are placed inside or after optical switches, such as the ones disclosed in References A and C, often causes sensitivity degradation of the optical parts on the reception side due to light surges caused by the above described interruption of light. Thus, the configuration disclosed in Reference A or C requires a surge-preventing function in the switching controlling unit of the optical switches and/or the controlling unit of the optical amplifiers. Otherwise, the disclosed high capacity switching apparatus needs to use high performance optical parts such as ones with a wide dynamic range. In addition, to compensate for the optical signals which suffer the power loss in the optical switches, the high-capacity switching apparatus includes optical amplifiers placed after the optical switches. Since the spontaneous emission noise of the optical amplifiers is added to the optical signals with a lowered power, the signal-to-noise ratio of the optical signal may decrease and cause errors in the receiver.
Furthermore, the optical signal received by the input port of an optical switch may take various inner paths before reaching the output port, and the optical switch in each stage is appropriately selected and configured. That is, because the characteristics such as the amplitude loss or the differential loss between the channels of each of the switches in the multi-stage combination is different, the loss between the channels of the optical switches between the input port and the output port will significantly vary depending on the actual configuration of optical switches in the multi-stage combination. Therefore, to offer a high performance large-capacity optical switch, it is desirable to realize compensation for the optical switch loss and the differential loss between channels that have occurred in the chosen optical path for each input/output port. The optical switching apparatuses as disclosed in Reference A or C, however, do not offer the above desired function.
Meanwhile, the OADM as disclosed in Reference B adopts a configuration wherein the spectra of wavelength-division-multiplexed optical signals are monitored and the loss is compensated for each demultiplexed wavelength in the OADM. In this configuration, since the wavelength of each signal to be compensated must be different from one another, the wavelengths and the multiplexing methods of the optical signals used as optical switching apparatuses will be limited. In addition, it is still not compatible with either an optical switch with a flexible configuration wherein the wavelengths monitored by the monitor units correspond to the wavelengths processed by the loss compensation units in a one-to-one fashion. It is desired various connections should be adopted with switching. Alternatively, an optical switch should have a flexible configuration with no restrictions in the wavelength of the optical signals in the multiplexing methods.